1. Field of the Invention
This invention relates to a toner for forming an image, and an image forming method and a heat fixing method, used in recording processes that utilize electrophotography, electrostatic recording, magnetic recording or the like. More particularly, this invention relates to a toner for forming an image, and an image forming method, used in an image forming apparatus such as copying machines, printers, facsimile machines and so forth in which a toner image is previously formed on an electrostatic latent image bearing member, the toner image is thereafter transferred to a transfer medium and the toner image transferred is fixed on a recording medium to form an image.
2. Related Background Art
A number of methods are conventionally known for electrophotography. Final images such as copies or prints are commonly obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner to make it visible to form a toner image, transferring the toner image to a transfer medium such as paper if necessary, and thereafter fixing the toner image to the transfer medium by heat or pressure.
As methods of visualizing an electric latent image, a cascade development method, a magnetic brush development method and a pressurizing development method are known. In another known method, a magnetic toner and a rotating sleeve with a magnetic pole disposed in a center thereof are used, so that the magnetic toner is allowed to fly in an electric field between a photosensitive member and the rotating sleeve.
As printers, LED printers and LBP printers are prevailing in the recent market. As a trend of techniques, there is a tendency toward higher resolution. That is, those which hitherto have a resolution of 240 or 300 dpi are being replaced by those having a resolution of 400, 600 or 800 dpi. Accordingly, with such a trend, the developing systems are now required to achieve a higher minuteness. Copying machines have also made progress to have higher functions, and hence they trend toward digital systems. The digital systems chiefly employ a method in which electrostatic latent images are formed by using a laser, and hence, the copying machines also trend toward a high resolution and, like the printers, it has been sought to provide a developing system with high resolution and high minuteness. For this reason, as a trend of techniques, there is a tendency toward smaller particle diameter of toner. Japanese Patent Application Laid-Open No. 1-112253 and No. 2-284158 propose toner having smaller particle diameter in the specific particle diameter distribution.
Recently, a tendency is thus proceeding toward a high resolution. However, since a toner circularity is not regulated, a density of developed toner particles tend to become coarse. There arises a problem that trailing phenomenon (i.e. smeared image trailing edge) easily occurs in a non-image portion in the latter half of a developed image. Further, an external additive cannot uniformly adhere to a toner surface. Therefore, another problem lies in shelf life (or storability) of the toner at high temperature and humidity.
In a general method of manufacturing the toner, a dye or a pigment as a colorant is molten and kneaded in a binding resin constituted of, e.g., a thermoplastic resin, and uniformly dispersed therein. Thereafter, a jet air current grinding is performed by using a jet air current. Especially, pulverizing is performed with a pulverizing device like an impingement air current pulverizer. Further, resulting powder is classified with a classifier to manufacture a powder having a desired particle diameter. The method is mainly used at present because of its mass productivity and cost effectiveness.
For example, when a toner with an average particle diameter of 6 .mu.m is obtained by using the pulverizer, in a particle size distribution of the pulverized toner, about 0.6 .mu.m to 10 .mu.m particles including multiple fine particles are distributed. In a classification process, the fine particles are removed before obtaining a toner product. However, ultrafine particles with a particle diameter of 1 .mu.m or less have a strong adhesion force to particles. The ultrafine particles behave while adhering to larger particles. Therefore, it is difficult to completely remove the ultrafine particles in a usual classification process.
Many proposals have been heretofore put forth to remove fine toner powder or suppress the generation thereof. However, it has been heretofore difficult to exactly measure the distribution of particle diameters of 1.0 .mu.m or less without being influenced by noises. Therefore, the toner ultrafine particles with particle diameters of 1 .mu.m or less are not clearly described. For example, in the Japanese Patent Application Laid Open No. 58-42057 and No. 6-317931, an objective fine powder is in the range of 5 .mu.m or less, and the toner ultrafine particles with particle diameters of 1 .mu.m or less are not clearly described.
When many ultrafine particles with particle diameters of 1 .mu.m or less are present, there is a large difference in toner electrification quantity between an initial condition and a long-run condition. Accordingly, there arises a phenomenon that a toner transferability is varied.
When the phenomenon arises, during formation of a full-color image a four-color toner image is not uniformly transferred. An irregular color or a problem about a color balance easily occurs. It is not easy to stabilize an output of the full-color image of a high quality.
Additionally, the ultrafine particles with particle diameters of 1 .mu.m or less are easily deposited on a toner carrier surface or a latent image bearing member surface. Further in the case where a resin with a low softening point is used or in another case, the deposited ultrafine particles tend to form a film, thereby causing an image defect.
Further, in recent years printing is performed at a high speed and fixing is performed with a low energy. Therefore, as a binder resin of a toner, a resin which softens at low temperatures is mainly used. The resin generally has a high grindability. Therefore, the ultrafine particles with particle diameters of 1 .mu.m or less tend to be easily generated. Further, since the resin softens at low temperatures, the particles tend to be easily deposited on the toner carrier surface or the latent image bearing member surface or to easily form a film.